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Using Spectrum ‘White Spaces’
The United States has long been a pioneer in the use of the ‘white spaces’ between broadcasts by users of unlicensed transmitter devices. Back in November 2008, the Federal Communications Commission (FCC) agreed to allow the introduction of unlicensed transmitter devices in white spaces and, in 2011, approved several white space databases and devices. However, since then, enthusiasm for white space usage in the US has been damped by discussions on the spectrum incentive auctions which may impact the amount of white space spectrum available in the UHF band.
Meanwhile, in the United Kingdom, the communications regulator Ofcom has been proceeding with its consultations on various aspects on white space usage. In 2011, it issued a statement which set out its approach for authorizing the use of white spaces. And just last week, Ofcom opened a consultation on the requirements for white space devices which included a list of features for devices, proposed parameters, a draft interface requirement (IR) document, and a draft voluntary national specification (VNS). Some analysts have predicted that transmitting devices making use of white spaces in the UHF band could be launched on the market as early as the end of 2013. However, it is still unclear what services will be offered by devices making use of the white spaces and what would be the associated business models.
Already, trials have been undertaken in Germany, Finland, Singapore, the United Kingdom, and the United States and further ones are planned in Brazil, Japan, and South Africa.
Understanding white spaces
White spaces are unused frequencies between active incumbent services at a given location. While they could be located anywhere on the radio-frequency spectrum, most national administrations are initially considering the white spaces in the UHF band that are located between broadcast television services. To avoid the risk of co-channel interference, high-power DTT transmissions require a large geographical separation from the next high power transmitter which reuses the same frequency channels. Adjacent transmitter areas use different groups of frequency channels and at a given location a significant number of frequency channels must be left unused. Since the active DTT channels in any given area do not change very often, it is relatively easy to predict which frequencies may be available for alternative, relatively low power, uses. It is these so-called white spaces, or interleaved spectrum, that service providers would like to be able to access.
According to potential service providers, including many multinational technology giants, white spaces offer the opportunity to offer innovative services. These include enhanced Wi-Fi, or "Wi-Fi on steroids" as it has been called in the United States, wireless broadband services in rural areas and machine-to-machine communications. Also known as the "Internet of Things", machine-to-machine communications can connect everyday objects to the Internet and, for example, allow utility providers to control utility meters or water levels from a physical distance.
Use of the white spaces in the TV UHF band is not new. Programme making and special events (PMSE) equipment traditionally operates in these frequencies, in particular for wireless microphones which are essential for broadcasting production and for live events such as theatre shows and music concerts. Large numbers of wireless microphones are also used in conference rooms, schools and universities, churches, trade shows etc.
Proposed plans for the use of white spaces by unlicensed transmitting devices have been contingent on the protection from interference of services provided by the incumbent licence holders, such as broadcasters and PMSE providers. Several technical solutions have been proposed that would allow unlicensed devices to be made aware of existing incumbent services and the availability of white spaces frequencies at a given location.
With beacon sensing technology, a beacon transmission is set up to protect the existing services by sending signals to the unlicensed device which inform it of the local activity in the spectrum. However, this technology has been deemed impractical compared to two other technologies, spectrum sensing and geo-location database, which are likely to be used in tandem over the next few years.
Spectrum sensing allows a white space device to detect the presence of the primary spectrum users and, hence, in turn enable it to identify the available unused frequencies at a given location. However, this technology is not yet considered sufficiently reliable if used on its own. Instead, it is used alongside a geo-location database which an unlicensed device accesses to determine which frequencies are permitted to be used at the given location and under which conditions. The database is updated regularly and, in the case of the United Kingdom, the database provider must adhere to certain obligations specified by Ofcom. In the United States, the database provider operates with a license from the FCC. Currently, unlicensed white space devices that only make use of sensing technology must be approved by the FCC on a case-by-case basis.
Implications for broadcasters
White spaces offer both challenges and opportunities to broadcasters. New opportunities are available to broadcasters who can make use of new technologies designed to operate in white spaces. For example, broadcasters could use the devices for their content production, in their news gathering, or as an additional option to deliver their content to viewers.
However, broadcasters want to ensure that any device operating in the white spaces does not cause interference to their DTT services. Some interference in some circumstances may be inevitable, but broadcasters must help determine how much is acceptable in terms of coverage loss at a given location. This could also affect portable reception which was an added inherent benefit in networks that were primarily planned for fixed, roof-top antenna reception.
To help with this challenge, the CEPT's Electronic Communications Committee (ECC) has set up working group SE43 to examine some of the technical issues related to white space usage in the UHF band. In 2011, SE43 prepared ECC Report 159 which defines the technical and operational requirements for white space devices while allowing for the protection of incumbent broadcast services.
Currently, SE43 is working on a complementary report which further defines the requirements for white space devices and addresses such issues as the approaches to setting power limits for white space devices and the impact of white space devices on services operating in the 800 MHz band. This report, to be known as ECC Report 186, is expected to be published in early 2013.
In addition, the standardization organization ETSI is currently developing a draft harmonised standard for white space devices operating in the UHF band.
Position of the European Commission
Last September, the European Commission (EC) issued a communication on promoting spectrum sharing in which it advocated the deployment of white space devices based on harmonised standards for geo-location databases.
While the EC suggests that the lower part of the UHF band from 470-698 MHz, which is currently used by the DTT platform, should initiate the use of the white space devices, it believes that white spaces should not be limited to a specific band. This may irk incumbent frequency users in other bands as it suggests that they may not be immune from the introduction of white space devices. A similar suggestion made by the United States President's Council of Advisors on Science and Technology (PCAST) for opening up of the 3.5-3.6 GHz band to white space devices raised concern with the wireless telecom industry group CTIA and the telecom operator AT&T.
Ultimately, however, the amount of white space available in the UHF band will be determined by the frequency capacity afforded to broadcasters. Current plans to reduce the DTT platform in the United States through the spectrum incentive auctions will simultaneously reduce the white space available for unlicensed devices. In Europe, as the 800 MHz band and possibly also the 700 MHz band will in the future be used for mobile communication services, DTT transmissions need to be moved to the lower parts of the UHF band. This will result in a more intensive use of the spectrum for DTT thus further reducing the amount of white spaces in the future.
For further information on using white spaces in Europe, please see two European research projects, COGEU and QUASAR, which were both co-financed by the European Union as part of its FP7 research programme.
Source: Natalie Mouyal, on behalf of the DigiTAG Project Office
DigiTAG aims to encourage and facilitate the implementation and introduction
of digital terrestrial television services using the Digital Video Broadcasting
Project's Standard (DVB-T). It has members from broadcasting,
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